Haskell on Windows - Stack fails to fetch package index - windows

Question

I'm trying to install Haskell on Windows. Downloaded the installer and just clicked through everything, then tried to use Stack to install a package, ran it from a temporary folder in which everything has write access:
C:\t>stack install hfmt
Using latest snapshot resolver: lts-8.3
Writing implicit global project config file to: C:\sr\global-project\stack.yaml
Note: You can change the snapshot via the resolver field there.
Downloaded lts-8.3 build plan.
Fetching package index ...=.git""=="gui" was unexpected at this time.
C:\sr\indices\Hackage\git-update\all-cabal-hashes>#if ""--git-dir=.git""=="gui" #goto gui
Process exited with ExitFailure 255: C:\Program Files (x86)\Git\cmd\git.CMD --git-dir=.git fetch --tags
Failed to fetch package index, retrying.
removeDirectoryRecursive: permission denied (Access is denied.)
What's going wrong, and how can I fix it? Or should I forget about Stack and just use Cabal instead?
Tried rerunning the command as administrator. This time the response was instant:
C:\t>stack install hfmt
Fetching package index ...=.git""=="gui" was unexpected at this time.
C:\sr\indices\Hackage\git-update\all-cabal-hashes>#if ""--git-dir=.git""=="gui" #goto gui
Process exited with ExitFailure 255: C:\Program Files (x86)\Git\cmd\git.CMD --git-dir=.git fetch --tags
Failed to fetch package index, retrying.
removeDirectoryRecursive: permission denied (Access is denied.)